block: Add bdrv_qapi_perm_to_blk_perm()

We need some way to correlate QAPI BlockPermission values with
BLK_PERM_* flags.  We could:

(1) have the same order in the QAPI definition as the the BLK_PERM_*
    flags are in LSb-first order.  However, then there is no guarantee
    that they actually match (e.g. when someone modifies the QAPI schema
    without thinking of the BLK_PERM_* definitions).
    We could add static assertions, but these would break what’s good
    about this solution, namely its simplicity.

(2) define the BLK_PERM_* flags based on the BlockPermission values.
    But this way whenever someone were to modify the QAPI order
    (perfectly sensible in theory), the BLK_PERM_* values would change.
    Because these values are used for file locking, this might break
    file locking between different qemu versions.

Therefore, go the slightly more cumbersome way: Add a function to
translate from the QAPI constants to the BLK_PERM_* flags.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20191108123455.39445-2-mreitz@redhat.com
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>